Production of phosphoric acid

ABSTRACT

Phosphate rock is reacted with hydrochloric acid in the presence of a water-soluble sodium compound to produce an aqueous acidulate containing phosphoric acid. The aqueous acidulate is extracted with a homogeneous organic liquid extractant comprising a liquid hydrocarbon and a low molecular weight, acidulateimmiscible alcohol to produce an organic solution of phosphoric acid. The organic solution of phosphoric acid is extracted with water and the resulting extract is concentrated to produce phosphoric acid which is low in calcium chloride content. Preferably, the organic solution of phosphoric acid is purified by extraction with aqueous phosphoric acid to remove calcium prior to extraction with water. The product phosphoric acid contains only 0.05-0.5 part by weight of calcium per 100 parts by weight of phosphoric acid, measured as P2O5, and is particularly useful in production of stable fertilizer solutions.

United States Patent [191 Klingelhoefer et al.

[ 1 Mar. 27, 1973 [54] PRODUCTION OF PHOSPHORIC ACID [75] Inventors:William Christian Klingelhoefer, Hopewell; James Earl Sansing, Jr.,

[21] Appl.No.: 100,367

Related US. Application Data [63] Continuation-impart of Ser. No.716,788, March 28,

1968, abandoned.

3,410,656 11/1968 Bunin et a1. ..23/165 Primary Examiner-Earl C. ThomasAssistant Examiner-Gregory A. Heller Attorney-Fred L. Kelly and PatrickL. Henry ABSTRACT Phosphate rock is reacted with hydrochloric acid inthe presence of a water-soluble sodium compound to produce an aqueousacidulate containing phosphoric acid. The aqueous acidulate is extractedwith a homogeneous organic liquid extractant comprising a liquidhydrocarbon and a low molecular weight, acidulate-immiscible alcohol toproduce an organic solution of phosphoric acid. The organic solution ofphosphor- [52] US. Cl. ..423/321, 23/312 D ic acid is extracted withwater and the resulting [51] Int. Cl. ..C0lb 25/16 tract is concentratedto produce phosphoric acid Field of Search 23/165, 165 B, 165 C, 312 Dwhich is low in calcium chloride content. Preferably, the organicsolution of phosphoric acid is purified by References Cited extractionwith aqueous phosphoric acid to remove calcium prior to extraction withwater. The product UNITED STATES PATENTS phosphoric acid contains only0.05O.5 part by weight 3,311,450 3/1967 Alon et al ..23/ 165 of calciumper 100 parts by weight of phosphoric acid, 3,397,955 8/1968 Champ eta1. ....23/ 165 measured as P 0 and is particularly useful in produc-2,880,063 3/1959 Baniel et a1 ....23/l65 {ion of stable fertilizersolutions. 2,954,287 9/1960 Carothers et a1. ..71/40 3,375,068 3/1968Frohlich ct a1. ..23/165 1 Claim, 2 Drawing Figures CONCENTRATEDHYDROCHLORIC ACID FEED ORGANIC ORGANIC OR4GANIC ORGANIC ORGANIC ORGANICEXTRACT EXTRACT EXTRACT EXTRACT EXTRACT EXTRACT I I I I LIQUID ORGANIC71 ll L [I ll ll T] I 11 'STAGE STAGE STAGE STAGE STAGE 7 l6 Is 3 2 1 IAQUEOUS H T Riggltsgii I PHASE I i AQUI'I'IOUS AQUOUS Aouzous AQU EOUSAQUOUS PHASE PHASE PHASE PHASE PHASE AQUEOUS ACIDULATE FEED STAGES I7 TO20 AQUEOUS REFLUX f AQUEQUS PHOSPHORIC ACID AQUEOUS AQUEOUS AQUEOUSAQUEOUS l f EXTRACT EXTRACT EXTRACT EXTRACT (fin r n WATER FEED STAGESTAGE STAGE STAGE 2| 22 26 27 R ZEFQIANPAITE ORGANIC ORGANIC ORGANICORGANIC ORGANIC EXTRACT PHASE PHASE PHASE PHASE PAIENIEDIIIIIZIIQH I v3,723, 06

SHEET 1 [IF 2 FIGI.

ACIDULATION OF PHOSPHATE ROCK WITH A REACTIVE EXCESS OF HYDROCHLORICACID IN THE PRESENCE OF SODIUM CHLORIDE SEPARATION OF SOLIDS FROM THEREACTION MIXTURE TO FORM A CLEAR AQUEOUS ACIDULATE CONTAINING PHOSPHORICACID AND CALCIUM CHLORIDE MULTISTAGE COUNTERCURRENT EXTRACTION OF THEAQUEOUS ACIDULATE WITH AN ORGANIC LIQUID EXTRACTANT COMPRISING A LOWMOLECULAR WEIGHT, ACIDULATE-IMMISCIBLE ALCOHOL AND A LIQUID HYDROCARBONIN THE PRESENCE OF HYDRO CHLORIC ACID, TO FORM AN ORGANIC SOLUTION OFPHOSPHORIC ACID MULTISTAGE COUNTERCURRENT EXTRACTION OF THE ORGANICSOLUTION OF PHOSPHORIC ACID WITH WATER TO FORM AN AQUEOUS SOLUTION OFPHOSPHORIC ACID CONCENTRATION OF THE AQUEOUS SOLUTION OF PHOSPHORIC ACIDBY EVAPORATION AND/OR DISTILLATION PRODUCTION OF PHOSPI-IORIC ACIDREFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of our copending application Ser. No. 716,788,filed Mar. 28, I968 and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the production oforthophosphoric acid (hereinafter referred to as phosphoric acid). Moreparticularly, the invention relates to the production of phosphoric acidby the reaction of phosphate rock with hydrochloric acid and extractionof the resulting aqueous acidulate. The phosphoric acid produced inaccordance with the present invention has low calcium salt content,particularly calcium chloride content.

It is known to produce and isolate phosphoric acid by acidulation ofphosphate rock with sulfuric acid followed by extraction of theresulting aqueous acidulate with certain polar organic liquids (e.g.,low molecular weight alcohols) which are capable of dissolvingphosphoric acid. It is also known to use hydrochloric acid as anattractive alternative to sulfuric acid in the acidulation step.However, when hydrochloric acid is used, the calcium chloride by-productis extracted from the aqueous acidulate in substantial quantities alongwith the phosphoric acid by the polar organic liquid extractant. As aconsequence, the phosphoric acid obtained as a final product iscontaminated with substantial quantities of calcium chloride. Importantapplications of phosphoric acid exist, e.g., in the fertilizer anddetergent arts, which require that the phosphoric acid used has a lowcalcium chloride content. Phosphoric acid is especially preferred foruse in the fertilizer and detergent arts when said acid contains no morethan about 1 part, desirably 0.05 to 0.5 part by weight of calcium per100 parts by weight of phosphoric acid expressed in terms of phosphoruspentoxide.

Accordingly, it is an object of the present invention to provide aprocess for producing phosphoric acid by the reaction of hydrochloricacid with phosphate rock, followed by isolation of phosphoric acid fromthe resulting aqueous acidulate, which phosphoric acid has a low contentof calcium salts, particularly calcium chloride.

Another object is to provide an improvement in the production ofphosphoric acid from phosphate rock, which improvement permits theproduction of phosphoric acid which has a low content of calcium salts,particularly calcium chloride.

Other objects and a fuller understanding of the present invention can behad by reference to the following detailed description and claims.

SUMMARY OF THE INVENTION In accordance with the present invention,phosphate rock is reacted or acidulated" with hydrochloric acid to forman aqueous solution (hereinafter referred to as aqueous acidulate oracidulate) containing principally phosphoric acid and calcium chloride.It is a feature'of the present invention that the acidulation ispreferably conducted in the presence of a water-soluble sodium compound.Concomitant with the formation of the aqueous acidulate there is formeda byproduct composed, inter alia, of siliceous material which isessentially insoluble in the aqueous acidulate. The aqueous acidulate isextracted with a homogeneous, water-immiscible solvent (hereinafterreferred to as organic liquid extractant) comprising a normally liquidhydrocarbon and a low molecular weight, acidulate-immiscible alcohol ormixture thereof. Preferably, the above-mentioned alcohol constitutesbetween about 65 percent by volume and about percent by volume of theorganic liquid extractant. The remainder of the organic liquidextractant is composed essentially of liquid hydrocarbon. In thismanner, an organic solution of phosphoric acid is formed as a liquidphase which separates from the now phosphoric acid-free aqueousacidulate (hereinafter referred to as aqueous raffinate). This organicsolution of phosphoric acid is in turn extracted with water to producean aqueous solution of phosphoric acid, which solution can beconcentrated with respect to phosphoric acid by conventional removal ofwater.

DESCRIPTION OF THE INVENTION The present invention is based upon thediscovery that when the organic liquid extractant is formulated tocontain a liquid hydrocarbon and a low molecularweight,acidulate-immiscible alcohol (e.g., isobutyl alcohol, isoamyl alcohol,and the like), the solubility of calcium chloride in the formulation isunexpectedly decreased to a much greater extent than the solubilitytherein of phosphoric acid. For example, isobutyl alcohol per seextracts about 1 part by weight calcium per 30 parts by weight P 0isoamyl alcohol per se extracts about 1 part by weight calcium per 50parts by weight P 0 However, a mixture of 75 percent by volume ofisobutyl alcohol and 25 percent by volume of a liquid hydrocarbon (e.g.,n-heptane, toluene, kerosene, and the like) extracts about 1 part byweight calcium per parts by weight P 0 a mixture of 67 percent by volumeof isobutyl alcohol and 33 percent by volume of liquid hydrocarbonextracts about 1 part by weight calcium per parts by weight P 0 In otherwords, the incorporation of a liquid hydrocarbon into the extractantincreases the selectivity thereof toward phosphoric acid. As a result ofthis hitherto unappreciated phenomenon, when an organic liquidextractant according to the present invention is employed in lieu of thelow molecular weight, acidulateimmiscible alcohol per se for purposes ofextracting phosphoric acid from the aqueous acidulate, the phosphoricacid so extracted can be obtained without substantial contaminationthereof with calcium chloride. Rather, the calcium chloride generallyremains in solution within the aqueous raffinate.

The present invention is also based upon the further discovery that,whereas low molecular weight, acidulate-immiscible alcohols suffer adecrease in extraction efficiency as the phosphoric acid concentrationof the acidulate decreases, mixtures of liquid hydrocarbon and saidalcohols on the other hand show only a relatively slight decrease inphosphoric acid extraction efficiency. This unexpected dichotomy betweenthe countercurrent 25 C. extraction power of low molecular weight,acidulate-immiscible alcohols and the extraction power of mixturesthereof with liquid hydrocarbons is exploited in the present inventionby using countercurrent extraction as the preferred mode of extractingthe aqueous acidulate with organic liquid extractant. In thisconnection, it is a feature of the present invention that an increase inthe number of stages of the countercurrent extraction results in anincrease in the selectivity of the. organic liquid extractant towardphosphoric acid. Accordingly, the purity (relative to calcium salts) ofthe phosphoric acid produced in accordance with the present inventionincreases with increasing number of stages in the counter-currentextraction. In particular, if the aqueous acidulate is extracted withorganic liquid extractant in a countercurrent extraction set-up havingat least about nine stages, the resulting organic solution of phosphoricacid generally contains about 1 part by weight of calcium per 100 partsby weight phosphorus pentoxide. Phosphoric acid of such high purity withregard to calcium salts is particularly useful in the manufacture offertilizers and detergents.

Thepresent invention has been described above in terms of phosphate rockas the starting material. However, to the present process (which can beconducted in either a batchwise or continuous manner) is applicable-toother materials (e.g., phosphatic chalk, guano, bone, and the like)which contain substantial amounts of calcium phosphates, particularlytricalcium phosphate Ca (PO In view of the natural abundance ofphosphate rock and the wide geographical distribution thereof, thepresent invention will be described in terms of phosphate rock as thepreferred starting material.

The step of acidulating phosphate rock with hydrochloric acid isgenerally conducted using minimum amounts of water to ensure arelatively high concentration of phosphoric acid in the aqueousacidulate. Preferably, concentrated hydrochloric acid (i.e., 37-38percent by weight aqueous I-ICl is employed. Alternatively, theacidulation step can be conducted by contacting an aqueous slurry ofphosphate rock with vapor-phase hydrogen chloride. In this connection,it is noted that the use of gaseous hydrogen chloride permits thecontrolled addition thereof to the aqueous slurry. As a result, foamingof the acidulation reaction mixture can be minimized. Furthermore, theacidulation step is preferably conducted in the presence of a reactiveexcess of hydrochloric acid; a hydrochloric acid equivalent excess ofbetween about 3% and about is especially preferred. Under theseconditions the insoluble by-product contains negligible amounts ofunreacted phosphate.

Phosphate .rock suitable for use in the present process can be of anyform heretofore employed in the acidulation art, although the efficiencyand rate of the acidulation generally increase as the average particlesize of the phosphate rock decreases. Accordingly, it is preferred touse phosphate rock in finely comminuted form.

The reaction of hydrochloric acid with phosphate rock proceeds rapidlyat ambient temperature. Preferably, however, a temperature of betweenabout 100 C. and about 1 15 C. is employed, whereupon the reaction isgenerally completed in about 1-15 minutes.

During the acidulation of phosphate rock (which contains substantialquantities of fluorapatite, Ca (PO )B2'CaF hydrogen fluoride is formedwhich causes silicates present in the phosphate rock to dissolve in theaqueous acidulate. This soluble silica, unless removed, causes severeemulsion problems during the extraction stages of the present process.The addition of alkali metal ion, particularly sodium ion, during theacidulation step causes precipitation of a major part of the solublesilica. Accordingly, a water-soluble sodium compound, e.g., sodiumchloride, sodium nitrate, sodium phosphate, sodium sulfate, sodiumcarbonate, and the like, is employed as the source of sodium ion.Desirably, between about 3 parts by weight and about 10 parts by weightof sodium compound, preferably sodium chloride, are incorporated witheach parts by weight of phosphate rock in the acidulation step. Theaddition of at least about 3 parts by weight of sodium chloride per 100parts by weight of phosphate rock during the acidulation according tothe present invention affords an aqueous acidulate containing no morethan about 0.1 percent by weight of dissolved silica (expressed interms'of SiO The use of larger quantities of sodium chloride results ina correspondingly lower concentration of dissolved silica in the aqueousacidulate. Insolubles can be removed from the aqueous acidulate byconventional means, e.g., d'ecantation, filtration, centrifugation, andthe like. It is also advantageous to treat the acidulate with settlingagents to hasten the separation of insolubles. The aqueous acidulate soobtained is a generally clear solution of phosphoric acid and calciumchloride, the concentration of which depends on the volume of wateremployed in the acidulation step. Ordinarily, the aqueous acidulatecontains about 5-15 percent by weight of phosphorus pentoxide and about20-40 percent byweight of calcium chloride. In addition, the aqueousacidulate can contain smaller amounts of hydrochloric acid and dissolvedcompounds of iron, aluminum, magnesium, silicon, and fluorine.

The aqueous acidulate is then extracted with an organic liquidextractant in the manner described hereinbelow. Prior to saidextraction, the water content of the acidulate can be adjusted so thatprecipitation of salts will not occur under conditions of extraction.Generally, the water content is adjusted according to the temperature ofthe extraction operation, a lower extraction temperature requiring ahigher acidulate water content, and vice versa. From the standpoint ofeconomy, it is desirable to conduct the extraction of the acidulate withas high a phosphoric acid concentration therein as possible.

The organic liquid extractant according to the present invention is ahomogeneous mixture of a liquid hydrocarbon and a low molecular-weight,acidulateimmiscible alcohol. The organic liquid extractant is, ofcourse, immiscible with water, but nevertheless capable of dissolvingphosphoric acid in substantial amounts.

Low molecular weight, acidulate-immiscible alcohols suitable forformulating the organic liquid extractant of the present inventioninclude all those capa ble of dissolving phosphoric acid and which arealso capable of forming homogeneous solutions with liquid hydrocarbons.Such alcohols generally contain between about four carbon atoms permolecule and about eight carbon atoms per molecule, viz., butanols,pentanols, hexanols, heptanols, and octanols. Illustra- Butanols andpentanols are the preferred groups of alcohols for use in the presentinvention, with n-butyl alcohol, iso-butyl alcohol and iso-amyl alcoholbeing especially preferred.

Liquid hydrocarbons suitable for formulating the organic liquidextractant of the present invention include all those capable of formingsolutions with the abovementioned low molecular weight,acidulate-immiscible alcohols. Such liquid hydrocarbons includealiphatic hydrocarbons (including saturated and unsaturatedhydrocarbons), carbocyclic hydrocarbons (including alicyclic andaromatic hydrocarbons), and mixtures thereof (e.g., turpentine,kerosene, gasoline, and the like). Illustrative of each of the abovegroups of liquid hydrocarbons are the following:

Aliphatic Hydrocarbons n-hexane and hexane isomers l,2-, and 3-hexenesn-heptane and heptane isomers n-octane and octane isomers, especially(i.e., 2,2,4-trimethylpentane) Carbocyclic Hydrocarbons (a) AlicyclicHydrocarbons cyclohexane cyclohexene cycloheptane cyclooctane decalin(b) Aromatic Hydrocarbons benzene toluene 0-, m, and p-xylenesethylbenzene 2-methylnaphthalene Liquid hydrocarbons which areespecially preferred for use in the present invention include hexanes,heptanes, benzene, toluene, ethylbenzene, and xylenes.

Preferred organic liquid extractants according to the present inventionare formulated to contain between about 65 percent by volume and about90 percent by volume of low-molecular weight, water-immiscible alcoholand, correspondingly, between about 10 percent by volume and about 35percent by volume of liquid hydrocarbon. Especially preferred organicliquid exiso-octane tractants are formulated to contain between aboutpercent by volume and about percent by volume of alcohol and,correspondingly, between about 15 percent by volume and about 25 percentby volume of hydrocarbon.

In producing phosphoric acid according to the present process, it isadvantageous to saturate the or- I ganic liquid extractant with waterprior to using said extractant in order to minimize volume losses fromthe aqueous acidulate during the extraction thereof.

The acidulate can be extracted with an organic liquid extractant in anymanner conventional to the liquidliquid extraction art. However, fromthe standpoint of achieving maximum extraction efficiency (i.e.,extracting a maximum amount of phosphoric acid with a minimum amount ofextractant), and product purity, it is preferred to conduct theextraction steps of the present process using multistage countercurrentextraction set-ups. In preferred embodiments of the present invention,the extraction of aqueous acidulate with organic liquid extractant andthe subsequent extraction of the resulting organic solution ofphosphoric acid with water are conducted inv multistage countercurrentextraction set-ups of at least about nine stages each. A particularlypreferred mode of carrying out the extraction steps of the presentprocess involves use of an extraction set-up as described hereinafter inconnection with FIG. 2. Such an extraction set-up permits the morecomplete extraction of phosphoric acid, the greater the number ofextraction stages employed therein. Preferably, the extraction set-upaccording to the present invention contains between about 20 and about40stages. Especially preferred is an extraction set-up containingbetween about 25 and about 30 extraction stages. Between about 3 partsby volume and about 20 parts by volume of the organic liquid extractantare used per part by volume of aqueous acidulate extracted therewith.Extraction set-ups involving more than about 40 stages are, of course,suitable for use in the present invention. However, such set-ups do notoffer any particular advantage over set-ups containing between about 25and about 40 stages.

It is preferred to conduct the extraction of aqueous acidulate withorganic liquid extractant at a uniform temperature of between about 25C. and about C., and particularly at a temperature of between about 40C. and about 80 C. It is also preferred to conduct the extraction in thepresence of strong acid to avoid the precipitation of insolublephosphates in the extraction set-up. Volatile mineral acid, especiallyhydrochloric acid, is preferred for such purpose due to the ease ofremoval thereof from the phosphoric acid product by evaporative means.Generally, the weight ratio of HCl added to the acidulate feed to P 0present therein is preferably maintained at least as high as about 0.9.

In a typical preferred multistage countercurrent extraction procedure,the organic liquid extractant of the present invention is introduced atone end of the set-up and aqueous raffmate is withdrawn at this stage.Mineral acid, preferably hydrochloric acid (concentrated hydrochloricacid is especially preferred) is introduced at one or more stagesbetween the acidulate feed point and the organic liquid extractant feedpoint. Aqueous acidulate is introduced at a stage near the middle of theextraction setup and aqueous phosphoric acid (containing hydrochloricacid) is withdrawn at a point several stages beyond this aqueousacidulate feed point. The intervening extraction stages are used forextraction of calcium from the organic liquid extractant. Water forextraction of the organic solution of phosphoric acid is introduced atthe last extraction stage and washed organic liquid extractant iswithdrawn at this stage.

The aqueous phosphoric acid solution obtained contains typically about12 percent by weight P 12 percent by weight HC! and less than 0.1percent by weight calcium. This aqueous solution can be concentrated toany desired P 0 content by conventional evaporation and distillationprocedures. Water, HCl, and minor amounts or organic solvent are takenoverhead and can be recovered and recycled. Generally, the aqueousphosphoric acid solution is concentrated to contain about 60 percent byweight P 0 BRIEF DESCRIPTION OF THE DRAWINGS In the accompanyingdrawings,

FIG. 1 is a diagrammatic flow sheet illustrating the process of thepresent invention in its broadest aspects.

FIG. 2 is a schematic illustration of a preferred mode of carrying outthe extraction steps of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 2, eachbox represents an individual stage of a preferredmultistagecountercurrent extraction set-up. Organic liquid extractant is added atstage 1 of the extractor and aqueous raffinate is withdrawn at'thispoint. Aqueous acidulate is added at stage 16. Concentrated hydrochloricacid is added at stage 2 to maintain high acidity during the extractionof the aqueous acidulate with the organic liquid extractant. Stages 17through 20 are used for reflux, i.e., purification purposes. Aqueousphosphoric acid containing HCl is withdrawn at stage 21, while water forextraction of the organic solution of phosphoric acid is introduced atstage 27. Washed organic liquid extractant is withdrawn from stage 27.

about nine to 16 stages at a temperature between about 25 C. and about80 C., said extraction further being conducted in the presence of atleast about 0.9 part by weight of added hydrochloric acid per. part byweight of P 0 in the acidulate feed, said hydrochloric acid being addedat the organic liquid feed end of said multistage countercurrentextraction, to produce an organic solution of phosphoric acid containinghydrochloric acid and about 1-6 parts by weight of calcium per 100 partsby weight of phosphoric acid, measured as P 0 d. extracting the organicsolution formed in step (c) with one-fifth to one-seventh part by volumeper part of organic solution, of aqueous phosphoric acid recycled fromstep (e), said extraction being conducted as a multistage countercurrentextraction of two to nine stages at a temperature between about 25 and80 C., to extract calcium from the organic solution and produce anorganic solution of phosphoric acid containing hydrochloric acid and0.05-0.5 part by weight of calcium per 100 parts by weight of phosphoricacid, measured as P 0 and an aqueous phosphoric acid solution containingextracted calcium;

e. extracting the organic solution formed in step (d) with aboutone-third to one-fifth part by volume per part of organic solution, ofwater, said extraction being conducted as a multistage extraction ofabout seven to An especially preferred process for the production ofphosphoric acid low in calcium-chloride content comprises:

a. reacting phosphate rock with a reactive excess of hydrochloric acidin the presence of a water-soluble sodium compound to produce an aqueousacidulate containing phosphoric acid, calcium chloride, and insolublesiliceous material;

b. separating the insoluble siliceous material from the aqueousacidulate to form a clear aqueous acidulate containing phosphoric acid,calcium chloride and no more than 0.1 percent by weight of dissolvedsiliceous material, expressed as SiO,;

c. extracting the clear aqueous acidulate with3-5 parts by volume perpart of acidulate, of a homogeneous, water-immiscible organic liquidextractant containing about 75 to 85 percent by volume of an alcoholselected from the group consisting of butyl alcohols and amyl alcohols,and about 25 to 15 percent by volume of a hydrocarbon selected from thegroup consisting of hexanes and heptanes, said extraction .beingconducted as a multistage countercurrent extraction of 14 stages at atemperature between about 25 and C. to produce an aqueous solution ofphosphoric acid containing hydrochloric acid and 0.05-0.5 part by weightof calcium per parts by weight phosphoric acid, measured as P 0 f.recycling part of the aqueous solution of phosphoric acid formed in step('e) to step (d); and

g. concentrating the residual aqueous solution of phosphoric acid fromstep .(e) by evaporation to remove the hydrogen chloride and part of thewater to form a concentrated aqueous phosphoric acid containing 0.05 0.5part by weight of calcium per 100 parts by weight phosphoric acid,measured as P 0 The aqueous phosphoric acid containing extracted calciumfrom step (d) is desirably fed to step (c) where it is commingled withthe aqueous acidulate for recovery of the phosphoric acid content by themultistage countercurrent extraction with the organic liquid extractant.

In the following examples, parts and percentages are by weight unlessotherwise indicated. Example I illustrates the application of thepresent process to aqueous acidulate formed by reacting phosphate rockwith concentrated hydrochloric acid. Example II illustrates theselectivity of the organic liquid extractant of the present inventiontoward phosphoric acid. Example III illustrates the criticality ofspecific operating conditions in the multistage extraction portion ofthe overall process.

EXAMPLE I One thousand parts of phosphate rock (through 30 U.S. mesh),analyzing 30.2% P 0 10.7% SiO,, and 45.1% CaO, are mixed with 1,694parts of concentrated hydrochloric acid, 465 parts of water, and 60parts of sodium chloride. The mixture is heated with conventionalagitation at 80-100 C. for 15 minutes. Agitation is then discontinuedand the solids are allowed to settle while maintaining the system at atemperature of about 85 C. The resulting hot, supernatant, aqueousacidulate is a clear solution of specific gravity 1.41 and analyzing 135grams P per liter, 145 grams calcium ion per liter, and 279 grams ofchloride ion per liter. The aqueous acidulate also analyzed 0.03% SiO Itis noted that, in runs identical to that described above but differingin that the sodium chloride is added after the acidulation and beforeextraction, the resulting aqueous acidulate analyzed 0.20.4% SiO Infurther runs, it is found that other water-soluble sodium compounds, e.g., sodium phosphate, sodium nitrate, sodium carbonate, can besubstituted for sodium chloride with equally good results.

The aqueous acidulate is next subjected to a multistage counter-currentextraction with an organic liquid extractant, followed by a multistagecounter-current extraction of the resulting organic solution ofphosphoric acid with water in a multistage counter-current liquid-liquidextractor represented schematically in FIG. 2. The temperature at eachstage of the extraction is maintained at between about 50 C. and about60 C. by conventional means (not shown). Organic liquid extractantcomposed of 4 parts by volume isobutyl alcohol for each part by volumeof n-heptane and saturated with water is introduced at stage 1 at therate of about parts by volume per hour; aqueous acidulate is introducedat stage 16 at the rate of about 5 parts by volume per hour;concentrated hydrochloric acid is introduced at stage 2 at the rate ofabout. 1.2 parts by volume per hour; water is introduced at stage 27 atthe rate of about 6 parts by volume per hour; and aqueous phosphoricacid analyzing 105 grams per liter P 0 117 grams per liter chloride ion,and 0.05 grams per liter calcium ion, is withdrawn at stage 21 at therate of about 7.1 parts by volume per hour. Aqueous raftinate analyzing1.4 grams per liter P 0 204 grams per liter chloride ion, and 114 gramsper liter of calcium ion is withdrawn at stage 1 at the rate of about6.7 parts by volume per hour.

The aqueous phosphoric acid withdrawn from stage 21 is concentrated toabout 60% P 0 content by evaporation, during which aqueous l-lCl isrecovered by condensation of the overhead.

The phosphoric acid product contains about 0.05 part by weight ofcalcium per 100 parts by weight of EXAMPLE II The following exampleillustrates the relatively high extraction efficiency of a preferredliquid hydrocarbonlow molecular weight alcohol mixture of the presentinvention as compared with the extraction efficiency of a low molecularweight alcohol per se. The preferred organic liquid extractant in thisexample is composed of n-heptane and isobutyl alcohol.

Data for this example are obtained by equilibrating, at 50 C., 20 partsby volume of the organic liquid extractant with 400 parts by volume ofaqueous acidulate prepared essentially as in Example 1 and adjusted tocontain 100 grams per liter of P 0 and 109 grams per liter of calcium.The composition of the organic phase at equilibrium is indicated inTable I.

TABLE I Composition of Organic Phase in Equilibrium with AqueousAcidulate at 50C.

Composition of Organic Phase 1 30 Calcium Organic Liquid (grams/ (grams/P 04 Extractant liter liter Calcium Isobutyl Alcohol 43 1.30 33 8:1 vol.ratio isobutyl 38 0.68 56 alcohol: heptane 4:1 vol. ratio isobutyl 310.38 82 alcohol: heptane The results summarized in Table I show that theweight ratio of P 0: to calcium increases with increasing proportion ofliquid hydrocarbon in the organic liquid extractant notwithstanding thecorresponding small decrease in P 0 concentration in the organic phase.This improvement becomes still greater using multistage extractiontechniques as illustrated in Example I.

EXAMPLE III In accordance with the preferred process, the overallextraction of phosphoric acid from acidulate involves three integratedextraction sections, which for convenience may be called extraction,purification, and recovery sections. Briefly stated, the extractionsection embodies multistage countercurrent extraction of the aqueousacidulate with the organic liquid extractant to form an organic solutionof phosphoric acid'containing small amounts of calcium. The purificationsection entails multistage countercurrent extraction of the organicsolution of phosphoric acid with aqueous phosphoric acid to extractcalcium into the aqueous phase and form an organic solution ofphosphoric acid containing less than 0.5 part by weight of calcium perparts by weight of phosphoric acid, measured as P 0 The recovery sectioninvolves multistage countercurrent extraction of the purified organicsolution of phosphoric acid with water to form a relatively pure aqueoussolution of phosphoric acid. Each section of the overall extraction hasbeen studied individually to ascertain criticality of processconditions. Operation of the entire extraction process as an integratedunit has also been studied. Results are reported below.

EXTRACTION SECTION Tests are carried out in a multistage countercurrentrocking funnel extractor using a solvent to acidulate volume ratio of5/ 1. The solvent is a solution of four volumes of isobutyl alcohol andone volume of heptane, said solution being saturated with water. Theacidulate is prepared in accordance with the procedure of Example I andcontains about -135 grams P 0 per liter. About one volume of 37 percentaqueous I-lCl per four volumes of acidulate is added near the solventfeed stage to improve extraction of phosphoric acid and avoidprecipitation of solids during the extraction.

It is found that about 95 percent of the phosphoric acid is extractedfrom the acidulate into the organic phase in four to five stages, andover 99 percent of the phosphoric acid is extracted in nine stages. Theaqueous HCl must be added at or near the raftinate removal stage ratherthan at the same stage in which the acidulate is fed to the extractionsystem. By so doing, the HCl can be maintained at a sufficiently highlevel throughout the extraction section to prevent precipitation.Extraction of phosphoric acid is relatively poor if the solvent toacidulate volume ratio is reduced to less than 3/1.

PURIFICATION SECTION Phosphoric acid and calcium are not completelyseparated in the extraction section. For example, the organic solutionof phosphoric acid from the extraction section can contain up to about 5parts by weight of calcium per 100 parts by weight of phosphoric acid.This ratio is greatly decreased by extraction of the calcium from thesolvent with aqueous phosphoric acid relatively low in calcium. Aportion of the purified phosphoric acid produced in the overallextraction operation is desirably used as extractant. This aqueousphosphoric acid typically contains about 80 to 140v grams P per literand has less than 0.5 parts by weight of calcium per 100 parts by weightof phosphoric acid, measured as P 0 Tests are carried out passing 30parts by volume of aqueous phosphoric acid product containing about 90grams P 0 per liter and having a P O zCa weight ratio of 200countercurrent to 200 parts by volume of organic extract from theextraction section, said organic extract initially having a P O :Caweight ratio of 20. Use of nine purification extraction stages gave anorganic extract having a P O :Ca weight ratio of 440; however, two tothree stages are adequate to reduce the calcium in the organic phase toless than 0.5 part by weight of calcium per 100 parts by weightphosphoric acid, measured as P 0 For the purification stages to fulfilltheir purpose, there must be sufficient aqueous phase being returnedcountercurrent to the organic phase, and a sufficient number of stages.A volume ratio of aqueous phase to organic phase of between aboutone-fifth and about one-seventh is desirable, and two to threepurification stages are preferred. In terms of recycle of product acid,the minimum ratio of product acid recycle to net product acid is about0.77 to l.

RECOVERY SECTION Phosphoric acid is recovered from the organic solventsolution from the purification section by being reextracted into anaqueous phase by countercurrent contact with water. A volume ratio oforganic phase to water of about 3/1 to 5/1 gives satisfactory recovery;e.g., about 98 percent recovery of phosphoric acid from the organicphase is obtained in lO-ll stages with an organic phase initiallycontaining 38 grams P 0, per liter. The highest practical concentrationof P 0 in the aqueous phase is about 175 grams P 0 per liter, about80-140 grams P 0 per liter being typical, with use of seven to 14washing stages.

Volume changes in the phases are appreciable, particularly in thepurification and recovery sections. In the purification section, up to80 percent of the aqueous phase has been observed to dissolve in theorganic phase, this aqueous phase being released in the recoverysection. Therefore, for convenience, ratios of the phases are expressedin terms of feeds to the sections.

Although addition of hydrogen chloride improves extraction of phosphoricacid from acidulate, hydrogen chloride makes recovery of phosphoric acidfrom the organic extract more difficult in the recovery section. Forthis reason, addition of hydrogen chloride to the extraction sectionshould be kept near the minimum that will avoid precipitation of solidsin the system. The weight ratio of hydrogen chloride added to theacidulate to the P 0 present therein is preferably about 0.9.

A major problem in prior extraction studies was that of silicaprecipitating in the recovery section and either giving emulsions orplugging the extractor. This problem has been overcome by removingsilica from the acidulate by adding sodium chloride to the reaction ofphosphate rock with hydrochloric acid.

COMPLETE EXTRACTION PROCESS The three sections of the completeextraction process, i.e., extraction section, purification section andrecovery section, are integrated into a single system, and performancedata are obtained using three solvents: isobutyl alcohol-heptane,isobutyl alcoholkerosene, and crude isobutyl alcohol-heptane, the crudeisobutyl alcohol containing about 10 percent by weight of lower alcoholsincluding isopropyl alcohol, ethanol and methanol. In one preferredcomplete extraction system, the extraction section consists of about15-1 6 stages; the purification section consists of two to three stages;and the recovery section consists of about seven to 10 stages. Theorganic solvent mixture contains 4 volumes of alcohol per 1 volume ofhydrocarbon. The acidulate is prepared in accordance with the procedureof Example I and contains grams P 0 per liter. The acidulate isextracted with about 3-5 parts by volume of the organic solvent.

Heptane and kerosene are compared as additives to isobutyl alcohol.Kerosene is not as satisfactory as heptane in the mixed solvent. Forexample, at each stage in the extraction section, the P 0 concentrationin the organic phase is about 14 percent higher in tests using theisobutyl alcohol-heptane than with the isobutyl alcohol-kerosenesolvent. Also, in the recovery section kerosene causesseparation intothree liquid phases in 5 stages including the product removal stage. Theappearance of three liquid phases is considered undesirable incommercial operation.

The crude isobutyl alcohol-heptane mixture is slightly less effectivethan the relatively pure isobutyl alcohol-heptane mixture with respectto separation of calcium from the phosphoric acid. Solvent loss is about10.0 percent with use of the crude isobutyl alcohol, probably due to therelatively water-soluble alcohols present in the system.

With use of the relatively pure isobutyl alcohol-heptane as solvent,weight ratios of P O zCa greater than 200:1 are easily attained in theproduct phosphoric acid.

In similar experiments a number of C to C alcohols and their mixtureswith hexane, heptane and toluene are tested as solvents. Mixturescontaining butyl alcohols and heptane or hexane give the best balancebetween effective separation of calcium and efficient extraction ofphosphoric acid. Using mixtures of isobutyl alcohol and heptane orhexane, improved separation of P 0 and calcium results at elevatedtemperatures of 50 C. or above, but more favorable extraction ofphosphoric acid from the acidulate is attained at 25-50 C., particularlyat ambient temperatures of about 25 C.

The foregoing examples are presented for the purpose of illustrating thenovel process of the present invention. It is, of course, understoodthat variations in the procedures described in those examples as well aschanges in the materials used therein can be made without departing fromthe scope of the invention. Other advantages over the prior art, notdisclosed herein may also exist for this invention which is defined inthe following claims.

What is claimed is:

1. A process for the production of phosphoric acid low in calciumchloride content, comprising:

a. reacting phosphate rock with a reactive excess of hydrochloric acidin the presence of 3 to 10 parts by weight of sodium chloride per 100parts by weight of phosphate rock to produce an aqueous acidulatecontaining phosphoric acid, calcium chloride, and insoluble siliceousmaterial;

b. separating the insoluble siliceous material from the aqueousacidulate to form a clear aqueous acidulate containing phosphoric acid,calcium chloride and no more than 0.1 percent by weight of dissolvedsiliceous material, expressed as SiO said clear aqueous acidulatecontaining 125-135 grams P per liter;

0. extracting the clear aqueous acidulate with 3-5 parts by volume perpart of acidulate, of a homogeneous, water-immiscible organic liquidextractant containing about 75 to 85 percent by volume of isobutylalcohol and about 25 to percent by volume of heptane, said extractionbeing conducted as a multistage countercurrent extraction of about nineto 16 stages at a temperature between about 25 C. and about 80 C., saidextraction further being conducted in the presence of at least about 0.9part by weight of added hydrochloric acid per part by weight of P 0 inthe acidulate feed, said hydrochloric acid being added at the organicliquid feed end of said multistage countercurrent extraction, to producean organic solution of phosphoric acid containing hydrochloric acid andabout l-6 parts by weight of calcium per 100 parts by weight ofphosphoric acid, measured as P 0 d. extracting the organic solutionformed in step (c) with one-fifth to one-seventh part by volume per partof organic solution, of aqueous phosphoric acid recycled from step (f),said extraction being conducted as a multistage countercurrentextraction of two to nine stages at a temperature between about 25 andC., to extract calcium from the organic solution and produce an organicsolution of phosphoric acid containing hydrochloric acid and about 0.05part by weight of calcium per parts by weight of phosphoric acid,measured as P 0 and an aqueous phosphoric acid solution containingextracted calcium;

e. recycling the aqueous phosphoric acid solution formed in step (d) tostep (c) for recovery of the phosphoric acid by multistagecountercurrent extraction with the organic liquid extractant;

f. extracting the organic solution formed in step (d) with aboutone-third to one-fifth part of water by volume per part of organicsolution, said extraction being conducted as a multistage extraction ofabout seven to 14 stages at a temperature between about 25 and 80: C. toroduce an aqueous solu tion of phosphoric aci contamlng hydrochloricacid and about 0.05 part by weight of calcium per 100 parts by weightphosphoric acid, measured as g. recycling part of the aqueous solutionof phosphoric acid formed in step (f) to step (d), the amount of saidacid recycled being at least 0.77 part per part of residual aqueoussolution of phosphoric acid; and

h. concentrating the residual aqueous solution of phosphoric acid fromstep (f) by evaporation to remove the hydrogen chloride and part of thewater to form a concentrated aqueous phosphoric acid containing about0.05 part by weight of calcium per 100 parts by weight phosphoric acid,measured as P 0

